The present invention relates to a photodetector and a unit mounted with a photodetector, and in particular relates to a photodetector that selectively receives signal light of a long wavelength (on the long wavelength side) when there is a plurality of incident signal lights of different wavelengths.
Currently there are broad applications for PIN photodiodes of a compound semiconductor material as photodetectors for optical fiber communications.
These PIN photodiodes employ a window structure to increase light-receiving sensitivity. In these PIN photodiodes, a light absorbing layer with a small forbidden bandwidth (long absorption edge wavelength) is formed on a semiconductor substrate, and a filter layer (window layer) with a large forbidden bandwidth (short absorption edge wavelength) is formed thereon. This structure makes it possible for light of the absorption edge wavelengths of the light absorption layer and the filter layer to be efficiently received.
A typical PIN photodiode is made of InGaAs/InP, where InGaAs is the material for the light absorption layer and InP is the material for the filter layer. In this case, the signal light in the wavelength range of 0.9 xcexcm to 1.65 xcexcm, which are the absorption edge wavelengths of InP and InGaAs, respectively, can be received with high sensitivity. An example of such a PIN photodiode structure is disclosed in Japanese Laid-Open Patent Publication No.1-238070.
Hereinafter, a conventional PIN photodiode will be described with reference to FIG. 10. FIG. 10 is a cross-sectional view of a conventional PIN photodiode.
As shown in FIG. 10, an nxe2x88x92-InP carrier blocking layer (buffer layer) 102, an nxe2x88x92-InGaAs light absorption layer 103, an nxe2x88x92-AlAsSb carrier blocking layer 104, and an nxe2x88x92-InP filter layer 105 are sequentially laminated onto an n+-InP substrate 101. Here, the forbidden bandwidth of the AlAsSb is larger than that of the InP, and has an absorption edge wavelength of 0.67 xcexcm.
A p+ diffusion region 106 made by the diffusion of Zn is formed in the filter layer 105 and the carrier blocking layer 104, and a surface protection film 107 is formed thereon. Additionally, a ringed anode electrode 108 is formed on the diffusion region 106, and a cathode electrode 109 is formed on the back face of the substrate 101.
It should be noted that since the PIN photodiode shown in FIG. 10 includes the carrier blocking layer 104, it has an improved sensitivity with respect to signal light of wavelengths of 0.9 xcexcm or less over a PIN photodiode in which the carrier blocking layer 104 is not formed.
To explain the reason for this, signal light of wavelengths of 0.9 xcexcm or less are absorbed by the filter layer 105 and generate electron-hole pairs, and in the case of PIN photodiodes in which the carrier blocking layer 104 is not formed, some of the electrons generated in the filter layer 105 flow into the light absorption layer 103 to become photoelectric current. In contrast, when there is the carrier blocking layer 104, the electrons are blocked by the hetero barrier at the interface between the filter layer 105 and the carrier blocking layer 104, and therefore do not become photoelectric current.
Passband type PIN photodiodes that are sensitive only to light of a narrower wavelength range have also been developed. For example, when performing wavelength multiplex communication with signal light of a 1.3 xcexcm wavelength and signal light of a 1.55 xcexcm wavelength, a passband type PIN photodiode sensitive only to those wavelengths is useful.
Here a PIN photodiode with short wavelength passband properties that is adequately sensitive to signal light of a 1.3 xcexcm wavelength but has almost no sensitivity to signal light of a 1.55 xcexcm wavelength can be readily achieved by forming the light absorption layer of the above PIN photodiode made of InGaAs, using an InGaAsP with an absorption edge wavelength of 1.4 xcexcm.
On the other hand, long wavelength passband properties where there is sufficient sensitivity to signal light of a 1.55 xcexcm wavelength but almost no sensitivity to 1.3 xcexcm wavelength signal light can be attained by taking InP as the material for the carrier blocking layer 104 and InGaAsP with an absorption edge wavelength of 1.4 xcexcm as the material for the filter layer 105. The relationship of the forbidden bandwidth of the carrier blocking layer 104 being larger than that of the filter layer 105 is maintained.
However, in conventional PIN photodiodes having passband properties, the filter layer 105 had to be made thick to increase the sensitivity difference with respect to long wavelength side signal light and short wavelength side signal light.
Accordingly, to achieve a PIN photodiode with long wavelength passband properties the material for the filter layer 105 should be InGaAsP, as described above. However, there is the problem that epitaxially growing a thick InGaAsP easily causes fluctuations in the composition of the InGaAsP.
On the other hand, in a planar PIN photodiode, in which impurities such as Zn are diffused in an island in the filter layer 105 to form a diffusion region, the thickness of the carrier blocking layer 104 is 1 to 2 xcexcm, and typically the diffusion of impurities is carried out to this depth.
This means that in conventional PIN photodiodes, even taking InGaAsP as the material for the filter layer 105 and being able to epitaxially grow the filter layer 105 thickly leaves the problem that it is difficult to diffuse impurities deeply and with good control. It should be noted that if the photodiode is a mesa PIN photodiode, in which the carrier blocking layer 104 and the filter layer 105 are doped so as to be p-type in advance and the element is separated by etching, it is no longer necessary to partially diffuse the impurities in an island-shape, but mesa PIN photodiodes, particularly those that are InGaAs/InP, have a large dark current and are less reliable than planar PIN photodiodes fabricated by impurity diffusion.
It is a main object of the present invention to provide a photodetector with long wavelength passband properties in which a high sensitivity ratio is obtained even with a thin filter layer and the depth of impurity diffusion in the light-receiving portion is equivalent to that of conventional PIN photodiodes.
A first photodetector according to the present invention includes a semiconductor substrate, a filter layer formed on a first principal face of the semiconductor substrate, a light absorption layer formed in an island on the filter layer, and a light incidence portion formed on the filter layer at a portion where the light absorption layer has not been formed; wherein the absorption edge wavelength of the filter layer is longer than the absorption edge wavelength of the semiconductor substrate and shorter than the absorption edge wavelength of the light absorption layer.
With the first photodetector, incident light that enters obliquely from the light incidence portion formed on the first principal face side of the semiconductor substrate is reflected at the second principal face so that the light passes through the filter layer, which is formed between the semiconductor substrate and the light absorption layer, twice before it is incident to the light absorption layer. This means that compared to photodetector with a structure in which the incident light passes through the filter layer a single time, the thickness of the filter layer can be made substantially double.
Additionally, it is preferable that in the first photodetector, a reflective film is formed on a second principal face of the semiconductor substrate.
Further, it is preferable that the first photodetector includes a carrier blocking layer formed between the filter layer and the light absorption layer, and of a material with a larger forbidden bandwidth than that of the filter layer; and a wide band gap layer formed on the light absorption layer and having an impurity diffusion region.
With this structure, the filter layer is between the light absorption layer and the semiconductor substrate, and thus a planar type PIN photodiode can be easily made by laminating a wide band gap layer on top of the light absorption layer to form a light-receiving portion by the diffusion of impurities. Moreover, the thickness of the filter layer can be made substantially double, so that the depth of impurity diffusion can be made equivalent to that of conventional PIN photodiodes. Furthermore, if a carrier blocking layer of a material with a larger forbidden bandwidth than that of the filter layer is interposed between the filter layer and the light absorption layer, then carriers generated at the filter layer can be prevented from flowing into the light absorption layer.
It is preferable that in the first photodetector, the semiconductor substrate, the filter layer, and the carrier blocking layer are of a first conduction type and the impurity diffusion region is of a second conduction type; and that the first photodetector includes a first electrode formed on at least one of the semiconductor substrate, the filter layer, and the carrier blocking layer; a second electrode formed on the impurity diffusion region; a pad portion formed on at least one of the semiconductor substrate, the filter layer, and the carrier blocking layer via an insulating film; and wiring for electrically connecting the second electrode and the pad portion.
With this structure, the p-side electrode and the n-side electrode can both be formed on the first principal face side of the semiconductor substrate, and flip-chip bonding can be easily performed. Also, by forming the pad for bonding the p-side electrode to the mounting substrate outside the island-shaped light absorption layer, the size difference between the light absorption layer mesa and the diffusion region can be reduced, so that tail current caused by incident light entering the light absorption layer aside from the portion thereof below the diffusion layer can be suppressed.
A second photodetector according to the present invention is characterized by further including in the first photodetector a sloped portion formed on a second principal face of the semiconductor substrate for reflecting incident light that has entered the semiconductor substrate and letting it incident to the light absorption layer.
With this structure, the incident light that is perpendicularly incident to the first principal face can be reflected by the sloped portion, so that even if the incident light is made perpendicularly incident, it can still be made incident to the light absorption portion.
A third photodetector according to the present invention includes a semiconductor substrate, a filter layer formed on a second principal face of the semiconductor substrate, a light absorption layer formed in an island on a first principal face of the semiconductor substrate, and a light incidence portion formed on the first principal face at a portion where the light absorption layer has not been formed; wherein an absorption edge wavelength of the filter layer is longer than an absorption edge wavelength of the semiconductor substrate and shorter than an absorption edge wavelength of the light absorption layer.
With this structure, like with the first photodetector, incident light that enters obliquely from the light incidence portion formed on the first principal face side of the semiconductor substrate is reflected at the second principal face so that the light passes through the filter layer, which is formed between the semiconductor substrate and the light absorption layer, twice before it is incident to the light absorption layer. This means that compared to a photodetector with a structure where the incident light passes through the filter layer a single time, the thickness of the filter layer can be made substantially double.
Further, it is preferable that the third photodetector include a wide band gap layer formed on the light absorption layer and having an impurity diffusion region.
In a fourth photodetector according to the present invention, the third photodetector further includes a sloped portion in the first principal face of the semiconductor substrate, wherein the light incidence portion is in the sloped portion.
With this structure, incident light that enters the first principal face perpendicularly can be refracted by the sloped portion, so that even perpendicularly incident light can be made incident to the light absorption layer.
Furthermore, it is preferable that in the first through fourth photodetectors, the light incident from the light incidence potion passes through the filter layer at least twice before it is incident to the light absorption layer.
A fifth photodetector according to the present invention includes a semiconductor substrate, a filter layer formed on a first principal face side or a second principal face side of the semiconductor substrate, a light absorption layer formed in an island on the first principal face side of the semiconductor substrate, a reflective film formed on the first principal face side of the semiconductor substrate, and a light incidence portion formed on the second principal face side of the semiconductor substrate; wherein the absorption edge wavelength of the filter layer is longer than an absorption edge wavelength of the semiconductor substrate and shorter than the absorption edge wavelength of the light absorption layer.
With this structure, the incident light enters from the light incidence portion formed on the second principal face side of the semiconductor substrate, but unlike typical rear-incidence type PIN photodiodes, where it is then incident to light absorption layer on the first principal face side, in this case it is reflected by the reflective film formed on the first principal face side and can be made incident to the light absorption layer after it has been reflected by the second principal face. In this case, the incident light passes through the filter layer three times, so that the thickness of the filter layer through which the incident light passes can be substantially tripled.
Further, it is preferable that in the fifth photodetector, the light incident from the light incidence portion passes through the filter layer at least three times before it is incident to the light absorption layer.
In a sixth photodetector according to the present invention, the filter layer and the carrier blocking layer are formed in a single or a plurality of islands, the semiconductor substrate is semi-insulating, the filter layer and the carrier blocking layer are of a first conduction type, and the impurity diffusion region is of a second conduction type; wherein the photodetector includes a first electrode formed on either the filter layer or the carrier blocking layer in the island having the light absorption layer, a second electrode formed on the impurity diffusion region, a pad portion formed on the semiconductor substrate, or on the filter layer or carrier blocking layer forming an island other than the island on which the first electrode has been formed, and wiring for electrically connecting the second electrode and the pad portion.
With this structure, by using a semi-insulating semiconductor substrate, the pad capacitance of the p-side electrode can be prevented from being added to the element capacitance.
A unit mounted with a photodetector according to the present invention has a mounting substrate, an optical wave guide formed within the mounting substrate, a polarizing element for polarizing the route of incident light propagating the optical wave guide to the surface direction of the mounting substrate, and a photodetector set to the mounting substrate; wherein the photodetector includes a semiconductor substrate, a filter layer formed on at least one of a first principal face side and a second principal face side of the semiconductor substrate, a light absorption layer formed in an island on the first principal face side of the semiconductor substrate, and a light incidence portion formed on the first principal face side of the semiconductor substrate; wherein the photodetector is set in such a way that the first principal face side on which the light incidence portion has been formed is directed toward the mounting substrate.
With this structure, the photodetector can be easily optically bonded from the mounting substrate side by flip-chip bonding, so that compared to the case where it is wire-bonded, there is a reduction in parasitic capacitance accompanying mounting and high operational speeds can be achieved. Moreover, the optical bonding is also completed at the moment of bonding, so that a mounting process for optical bonding is unnecessary.
Additionally, it is preferable that in the unit mounted with a photodetector the optical wave guide is an optical fiber buried in a groove formed on the mounting substrate, the polarizing element is a reflective plate inserted into a slit formed on the mounting substrate such that it severs the optical fiber, and the photodetector is set to the mounting substrate such that it spans the groove.
According to the photodetector of the present invention, incident light that enters from the light incidence portion is reflected by the first or second principal face and passes through a filter layer formed between a semiconductor substrate and a light absorption layer at least twice before it is incident to the light absorption layer. Thus, compared to photodetectors with a structure in which incident light passes through the filter layer only once, the thickness of the filter layer can be made substantially at least double. The result is that a sensitivity ratio of 20 dB or more, for example, can be obtained for signal light of a long wavelength side (1.55 xcexcm) and signal light of a short wavelength side (1.3 xcexcm).
Moreover, according to the unit mounted with photodetector of the present invention, the photodetector can be easily flip-chip bonded to the mounting substrate, so that compared to the case where it is wire-bonded, there is a reduction in parasitic capacitance that accompanies the mounting, high operation speeds become possible, and furthermore the optical bonding is also completed at the moment of bonding, so that a mounting step for optical bonding is unnecessary.